US2268505A - Signal selection in radio receivers - Google Patents

Description

1941- H. A. 'FAIRHURST ,268,505

SIGNAL SELECTION IN RADIO RECEIVERS Filed June 25, 1940 2 Sheets-Sheet l FIGI.

Dec. 30, 1941.

-l. A. FAIRHURST 2,268,505 SIGNAL SELECTION IN RADIO RECEIVERS Filed June 25. 1940 2 Sheets-Sheet 2 A T TOR/V5 Y Patented Dec. 30, 1941 SIGNAL SELECTION IN RADIO RECEIVERS Harold Alfred Fairhurst, Welwyn Garden City, England, assignor to Murphy Radio Limited,

London, England Application June 25, 1940, Serial No. 342,315 In Great Britain June 23, 1939 8 Claims.

This invention relates to radio receiving apparatus. Its purpose is to enable the suppression of undesired signals of short duration without materially affecting the transmission along the signal channel of the signals it is intended.

to pass.

A particular instance of this purpose is the suppression in a radio telephone receiver of the type of interference caused by the ignition systems of motor cars, that is to say, interference of the nature of impulses of less duration than the period of the highest audible note or harmonic it is desired to receive, these impulses often recurring at an audio frequency. The invention is particularly concerned with this problem as it arises in receivers working on very short wavelengths such as the sound receivers of television apparatus.

The invention rests on the differentiation of the undesired signal or noise from wanted signals by the short duration of its component impulses, and aims to remove any such noise by shunting brief pulses which rise above the instantaneous level of the wanted signal.

It is common practice to reduce the effects of such interference in radio receivers by the use of some form of limiter circuit, designed to limit to a fixed maximum value the output produced by all signals applied to it.

Such limiting circuits applied to telephony receivers must be so adjusted as. to pass the wanted modulation without distortion at all levels up to that corresponding to 100% modulation of the carrier wave. Hence they can limit noise impulses only to a corresponding amplitude. Since the average level of modulation in normal broadcast transmission is of the order of the interference let through will often considerably exceed the instantaneous amplitude of the wanted signal.

A principal object of the present invention is to suppress interference impulses of the kind above described without the loss of wanted signal that must inevitably take place along a normal conductive limiter shunt.

In a radiotelephony receiver according to the present invention, the bias applied to the limiter, which determines the level to which the output signal is limited, is an independent bias (as distinct from a self-bias or autobias) which includes an audio frequency component adjusted to substantial equality of amplitude with the signal at the point at which the limiter is branched from the signal channel so that the limiter is non-conductive to wanted signals free of interference pulses, and this bias voltage is applied to the 1imiter through circuits which have a slight delaying effect. The difference in phase thus brought about should be a small fraction of the periodic time of the highest frequency oscillations it is desired to deal with, but should preferably be not less than the-duration of an interfering noise pulse.

The characteristic of short duration of the component impulses of noise, on which the differentiation depends, must be preserved up to the audio-frequency stage in which elimination of noise is effected; and therefore the preceding radio frequency and intermediate frequency circuits must respond over a Wave band of sufiicient width for this purpose. The band width should be as large as is consistent with reasonable amplification and other competing considerations, and usually at least five times as wide as is called for by the highest audio frequency to be reproduced. A convenient form of limiter, suitable for the purpose of this invention, consists of a simple diode shunted across the input of an audio-frequency stage. The signals to be differentiated are applied to both electrodes of the diode, to the anode directly, to the cathode.

with slight delay. The cathode input may be taken from theoutput of the succeeding audio frequency stage in which case the delay circuit may be in either the input or the output circuit of that stage; or it may be taken from a triode preferably used as a cathode follower, forming a branch from the signal channel in which case the delay circuit may be in either the input or output circuit of the triode. The delaying circuit may be built up of series resistances and shunt condensers, or inductances may be used.

Typical circuits embodying the invention are illustrated in the accompanying drawings.

Figure 1 shows the diode cathode fed from the output of the succeeding audio frequency stage.

Figure 2 shows a modification of the scheme of Figure 1 providing for desired tone compensation.

Figure 3 shows the diode cathode fed from a cathode-follower triode forming a branch from the signal channel.

Figure 4 shOWs a modification of the scheme of Figure 3 in respect of the means for applying the appropriate-D. C. voltage to the anode of thediode.

Figure 5 shows a further modification of this scheme in respect of the input to the cathodefollower triode.

For the purpose of explaining the attainment of the principal object of the invention it may be assumed that the input terminals l in each figure receive audio frequency energy from a detector D or audio frequency amplifier of any usual type and supply it to the control grid of an output valve 2 from which a loud speaker, not shown, is fed in usual manner, for instance through the transformer 3 of Figuresliand 2.

For the purpose of the invention the input circuit of the valve 2 is shunted by a diode 4. To the cathode of this diode there is applied a bias comprising a D. 0. component intended to bring the diode to the verge of conducting and an audio frequency component intended to make the cathode potential closely follow the potential of the anode for all signals it is desired to,re-

ceive.

The signal input is applied to the anode of diode 4 and to the control grid of valve 2 through a condenser 5 and resistance 6. The grid leak l of valve 2 is returned to a tapping on the condenser-shunted cathode resistance 8 by which the valve is self-biased. D. C. bias for the diode 4 may conveniently be taken from a potentiometer 9 bridged across the cathode resistance 8, the tapped part of the potentiometer being shunted by a condenser II and connected in series with the secondary winding of transformer 3 between chassis and the cathode of diode 4. The connection is not direct but is made through a delaying network shown as built up of resistances l2 and condensers I3. The amplitude of the audio frequency voltage so applied to the cathode is adjusted to be equal to the amplitude of the voltage applied to the anode of the diode; its phase is shifted slightly by the delaying network; the delay should be not less than the expected duration of the short impulses it is desired to obliterate.

. When normal signals Within the desired range of modulation frequencies reach the anode of the diode 4 they are applied nearly simultaneously to the cathode also and the cathode remains non-conducting. These signals therefore pass along the signal channel without appreciable distortion. Short impulses of interference, on the other hand, whose duration is small compared with the time period of the highest audio frequencies it is desired to pass through the signal channel, will reach a sufiicient value upon the anode of the diode to render the anode conductive before they can begin to appear on the cathode.

Such impulses are therefore shunted and do not reach the grid of valve 2. Even should a second interfering impulse reach the anode of the diode at the instant when the first, delayed by the network [2, I3, would (save for the shunting effect) have reached the cathode it will still not pass because the first impulse having been bye-passed to chassis does not reach the oathode at all.

It is commonly desired in radio telephony receivers to make provision in the input of the output valve for reducing the bass response because it is likely to be unduly accentuated by loud speaker resonances. If any such compensation were attempted with the circuit of Figure 1, by placing a series condenser and shunt resistance in the grid circuit of valve 2, its effect would be to produce an appreciable phase displacement between voltages of low audio frequency applied to the anode of the diode and the corresponding voltages applied to the oathode, with the result that the diode would exercise its shunting effect and cause distortion of the desired signal.

If such reduction of the bass is attempted, therefore, in the input to the grid of valve 2, it must be compensated by a corresponding enhancement of the bass in the voltage applied to the cathode of the diode 4. This is illustrated in Figure 2 where a condenser I4 and resistance [5 in the grid circuit of valve 2 cause a reduction of the bass in the signals transmitted through the transformer 3, while the resistance I6 and condenser l! in the cathode circuit of the diode 4 cause a compensating increase of the bass in the voltage applied to the cathode.

Additional turns would be needed on the secondary of transformer 3 to allow for the voltage drop in this compensating network.

It will be apparent that the compensating network IG, I1 is essentially of the same nature as the delaying network l2, l3, and therefore, as indicated by Figure 2, a single network may be designed to serve both purposes.

It is, however, diflicult to adjust the phase relationship between the voltages on the cathode and anode of the diode in this circuit, especially for the lower audio frequencies. It is therefore preferable to supply the. cathode of the diode from an additional valve branched from the signal channel. This is shown in Figure 3. The valve 18, shown as acting as a cathode follower, is fed through the condenser l9 from the same point as the diode 4; its grid leak 2| is returned to a suitable point on its cathode resistance 22. The input to the valve is therefore unaffected by the bass-reducing circuit I4, l5 of the valve 2. The resistances 6, l are adjusted to produce the same audio frequency voltage on the anode of the diode 4 as appears on the cathode of valve l8. The requisite D. C. bias for the diode anode is got by adjusting its anode potential in relation to the potential of the cathode of valve I 8 by a potentiometer 23. The requisite delay in the voltages fed to the diode cathode is obtained from a delaying circuit shown, in this instance, as consisting of series inductances 24, coupled in pairs, and shunt condensers 25 forming a low pass filter completed by a terminating resistance 26. Such a circuit can be designed to give a sharp cut-off at a frequency above any desired audio frequency and below the eifective frequency of the interference.

As the voltage across the diode under optimum conditions is nearly zero the circuit may be simplified as shown in Figure 4 by the omission of the potentiometer 23, the voltage on the oathode of the cathode follower valve [8 being applied instead to the anode of the diode 4 through a suitable smoothing circuit comprising resistances 21 and capacitance 28.

It will be noted that in the circuits of Figures 3 and 4 all noise impulses applied to the anode of the diode 4 and the grid of valve 2 will appear, after the delay imposed by the network 24, 25, 26, upon the cathode also. If, therefore, a second noise impulse should follow the first at an interval approximately corresponding with this delay the second impulse might appear on the anode of the diode simultaneously with the appearance of the first impulse on the cathode, and would then not be obliterated.

This disadvantage may be obviated by feeding the grid of valve l8 as Well as the anode of the diode 4 from the junction of the resistances B and 21 as shown in Figure 5. This will make the audio frequency voltage on the cathode of the diode slightly less than that on the anode, but if the cathode load is made considerable the difference can be reduced to negligible proportions. The time constant of the input circuit 5, 6 is such that the circuit passes low frequencies undistorted. Since any delay greater than the duration of an interfering pulse is suflicient to prevent the building up of the pulse upon the anode of the diode and the grid of valve I8, the

value of the time constant of the delay network 24, 25 is not critical, and may be designed with reference to the frequency response desired in the output.

While the circuits above described are primarily designed for the elimination of interference of the kind first above noted from telephony signals it will be apparent that they can be made to separate any short impulses from longer impulses. For example such a circuit as is shown in Figure 3 may be designed to eliminate line synchronising impulses from a mixture of line and frame synchronising impulses in television reception. The grid of the valve of the usual saw-tooth generator is supplied from the anode of the diode 4, and the delay circuit 24, 25 is designed to produce delay exceeding the duration of a line synchronising impulse, but less than the duration of a frame synchronising impulse; thus the latter but not the former will reach the grid of the dis-charge valve.

I claim:

1. In a radio receiver the combination with carrier frequency circuits having a width of response in excess of that needed for reproduction of the highest audio frequency desired and a rectifier, of an audio frequency amplifier stage fed through said rectifier from said carrier frequency circuits, a diode shunting the input circuit of said audio frequency amplifier stage, and means for applying directly to the anode of said diode and through a delaying circuit to its cathode signal voltages adjusted to substantial equality of amplitude, said delaying circuit introducing delay approximately equal to the duration of expected interfering pulses.

2. In a radio receiver, means for separating signals of long duration from other signals of short duration comprising an amplifier including a grid-controlled thermionic tube to which said signals are fed, a limiter circuit fed in parallel with the control grid of said amplifier, and means for introducing into said limiter circuit an audiofrequency bias derived from and varying with the signals and adjusted to substantial equality of amplitude with the signal fed to the limiter circuit, said means including a delaying circuit which retards said bias relatively to the signals by the duration of the signals of short duration.

3. In a radio receiver the combination with carrier frequency circuits having a width of response large compared with the range of audio frequencies to be reproduced, a rectifier supplied from said carrier frequency circuits, and an amplifier including a grid-controlled thermionic tube fed from said rectifier, of a diode having its anode also fed from said rectifier, a tone-adjusting circuit included in the input circuit of said amplifier, and means for applying to the cathode of said diode a bias derived from the output of said amplifier, said means including a delaying and tone adjusting network to retard said bias relatively to the input to the diode and to compensate for the tone-adjustment effected by the tone-adjusting circuit.

4. In a radio receiver the combination with a signal channel comprising carrier frequency circuits having a width of response large compared with the range of audio frequencies to be reproduced, a rectifier supplied from said carrier frequency circuits, and an amplifier including a grid-controlled thermionic tube fed from the said rectifier, of a diode having its anode also fed from said rectifier, a thermionic tube fed from said rectifier as a branch from said signal channel, and means for applying to the cathode of said diode a bias derived from the cathode circuit of said branched thermionic tube, said means including a delaying network which retards said bias relatively to the input to the diode anode by an interval less than the periodic time of the highest audio frequencies to be reproduced.

5. In a radio receiver the combination with carrier frequency circuits having a width of response large compared with the range of audio frequencies to be reproduced, a rectifier supplied from said carrier frequency circuits, and an amplifier including a grid-controlled thermionic tube fed from the said rectifier, of a diode having its anode also fed from said rectifier, a cathode-follower tube fed from said rectifier, and means for applying to the cathode of said diode a bias derived from the cathode circuit of said cathode-follower tube, said means including a delaying network which retards said bias relatively to the input to the diode anode by an interval less than the periodic time of the highest audio frequencies to be reproduced.

6. In a radio receiver the combination with carrier frequency circuits having a width of response large compared with the range of audio frequencies to be reproduced, a rectifier supplied from said carrier frequency circuits, and an amplifier including a grid-controlled thermionic tube fed from the said rectifier, of a diode having its anode also fed from said rectifier, a cathode-follower tube fed from the anode of said diode, and means for applying to the cathode of said diode a bias derived from the cathode circuit of said cathode-follower tube, said means including a low-pass filter adapted to cut off above the highest audio frequency to be reproduced.

7. In a radio receiver, means for separating signals of long duration from other signals of short duration comprising an amplifier to which said signals are fed, a limiter circuit connected in parallel with the input circuit of said amplifier, a source of bias independent of the conductivity of said limiter circuit for deriving from the signal and applying to said limiter circuit an audio frequency component adjusted to substantial equality with the signal at the point at which the limiter circuit is branched from the signal channel, and delaying means by which said bias is retarded relatively to the signal.

8. A radio telephony receiver having in combination with an audio frequency amplifier included in the signal channel, a diode connected in parallel with the input circuit of said amplifier, and a bias circuit independent of the conductivity of the diode circuit for deriving from the signal and applying to the cathode of said diode a bias of audio-frequency adjusted to substantial equality of amplitude with the signal voltage applied to the anode of the diode thereby rendering said diode non-conductive for wanted signals, said bias circuit including a delaying network which retards the bias relatively to the signal by a fraction of the periodic time of the highest audio frequency to be reproduced.

HAROLD ALFRED FAIRHURST.

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